What is believed to be the second most common element in the universe?

The total number of isotopes of hydrogen and the number of radioactive isotopes among them,respectively,are:

If an isotope of hydrogen has two neutrons in its atom,its atomic number and atomic mass number will respectively be

Among the following,the correct statements are:
$I$. $LiH, BeH_2$ and $MgH_2$ are saline hydrides with significant covalent character.
$II$. Saline hydrides are volatile.
$III$. Electron-precise hydrides are Lewis bases.
$IV$. The formula for chromium hydride is $CrH$.
The correct option is:

Which of the following processes can be used to obtain heavy water $(D_2O)$?

Consider the following sequence of reactions: $Na + NH_{3(g)} \to [X]$ $\xrightarrow{N_2O} [Y]$ $\xrightarrow{\text{Heat}} [Z] (\text{Gas, Pure})$. Identify $[Z]$ gas.